The present invention relates to a method for making alkylhalosilanes. More particularly, the present invention relates to a process involving the reaction of methyl chloride and powdered silicon in the presence of a copper-zinc-tin catalyst.
Prior to the present invention, methylchlorosilanes were made by effecting reaction between powdered silicon and methyl chloride in the presence of a copper catalyst, as shown by U.S. Pat. No. 2,380,995, Rochow, assigned to the same assignee as the present invention. Improved results were achieved by utilizing a fluidized bed reactor, as shown by Reed et al, U.S. Pat. No. 2,389,931, also assigned to the same assignee as the present invention. Further improvements in the production of particular methylchlorosilanes were achieved when zinc was used in combination with copper catalyst as a promoter as shown by Gilliam, U.S. Pat. No. 2,464,033. Gilliam teaches that a proportion of from about 2 to about 50% by weight of copper in elemental form or as the halide or oxide, and preferably 5 to 20% and from about 0.03 to about 0.75% by weight of zinc in the form of zinc halide, zinc oxide, or zinc metal, or mixture thereof, where the weight of copper and zinc are based on the weight of silicon, can be used as a promoter for making dialkyl substituted dihalogenosilanes, such as dimethyldichlorosilane in the direct reaction between silicon powder and methyl chloride.
Subsequent to the investigation made by Gilliam, Radosavlyevich et al, found that micro quantities of silver added to contact masses resulting from the reaction of powdered silicon and methyl chloride in the presence of cuprous chloride decreased the yield of methylchlorosilanes, while tin and calcium chloride increased the rate of formation of methylchlorosilanes as reported in "Influence of Some Admixtures on the Activity of Contact Masses for Direct Synthesis of Methylchlorosilanes", Institute of Inorganic Chemistry, Belgrade, Yugoslavia, (1965).
As utilized hereinafter, the term "methylchlorosilanes" includes dimethyldichlorosilane, which is the preferred methylchlorosilane, and a variety of other silanes such as tetramethylsilane, trimethylchlorosilane, methyltrichlorosilane, silicon tetrachloride, trichlorosilane, methyldichlorosilane and dimethylchlorosilane.
In addition to the above methylchlorosilanes, "residue" is also formed during the production of methylchlorosilane crude. Residue means products in the methylchlorosilane crude having a boiling point &gt;70.degree. C. at atmospheric pressure. Residue consists of such materials as disilanes, for example, symmetrical 1,1,2,2-tetrachlorodimethyldisilane, 1,1,2-trichlorotrimethyldisilane, disiloxanes, disilmethylenes and other higher boiling species, for example, trisilanes, trisiloxanes, trisilmethylenes, etc. In addition to residue, those skilled in the art also are interested in T/D weight ratio of the methylchlorosilane crude. The T/D ratio is the ratio of methyltrichlorosilane to dimethyldichlorosilane in the crude methylchlorosilane reaction product. Accordingly, an increase in the T/D ratio indicates that there is a decrease in the production of the preferred dimethyldichlorosilane.
Although as taught by the prior art, while zinc or tin can be valuable promoters for copper catalyst, or copper-silicon contact mass in the reaction between powdered silicon and methyl chloride, it has been found that the rate of crude methylchlorosilane formation and the T/D ratio are often unsatisfactory.
When defining a rate constant for crude methylchlorosilane formation, the term "K.sub.p ", or "reaction rate constant for methylchlorosilane product" is often used by those skilled in the art. A more detailed derivation of K.sub.p, is shown below immediately prior to the examples.
K.sub.p values can be obtained using an apparatus as shown by FIG. 2. On a relative scale, when a mixture of copper and powdered silicon containing 5% by weight of copper is used to make methylchlorosilane, a "K.sub.p 38 (grams silane/grams silicon, hr) having a numerical value of about 13 can be obtained. A K.sub.p of 16 can be obtained from a mixture of powdered silicon containing 5% by weight copper and 0.5% by weight zinc. A K.sub.p of 45 can be obtained when a mixture of powdered silicon containing 5% weight copper and 0.005% by weight tin is reacted with methyl chloride.
Although the above K.sub.p values indicate that tin promoted copper catalyst can provide a superior methylchlorosilane formation rate when used with powdered silicon and methyl chloride, it has been found that the selectivity of tin promoted copper catalyst can be inferior to zinc promoted copper catalyst.
As defined hereinafter, the term "selectivity" means the ability of a catalyst to maximize the production of dimethyldichlorosilane, as shown for example by a reduction in the value of the T/D ratio and a reduction in the % residue. It is found, for example, that although a higher K.sub.p can be obtained when tin is utilized with copper to catalyze the reaction between powdered silicon and methyl chloride, a significant increase in the T/D ratio is also effected, as compared to the use of a copper catalyst promoted with zinc.
The present invention is based on the surprising discovery that direct method reactions between powdered silicon and methyl chloride in the presence of an effective amount of a copper-zinc-tin catalyst as defined hereinafter, can provide K.sub.p values about twice that shown for tin promoted copper catalyst as discussed above, while simultaneously substantially improving the selectivity over zinc promoted copper catalyst and over tin promoted copper catalyst. More particularly, optimum selectivity and maximum K.sub.p values can be obtained by practicing the direct method with a mixture of powdered silicon, copper, tin and zinc containing 0.5-10% by weight of copper, based on the weight of silicon, where the copper can be in the free state or in the form of a copper compound as defined hereinafter, with 0.01 to 0.5 part of zinc, per part of copper and 200-3,000 ppm of tin relative to copper, where both zinc and tin are also expressed in terms of weight of metal, although optionally employed as a zinc compound or a tin compound, as defined hereinafter.